1. Field of the Invention
The present invention relates to a temperature-compensated radio-frequency oscillator that achieves high frequency stability by providing temperature compensation for frequency-temperature characteristics of a SAW resonator.
2. Description of Related Art
An oscillator is mounted on a communication device such as a mobile phone. Such a device performs modulation and demodulation of communication data based on an output signal from the oscillator. Recently, there are demands for a communication device that achieves stable oscillation (high frequency stability) in the radio-frequency band (from several hundred MHz to several GHz) and stable oscillation in the range of practical temperature for the communication device (temperature compensation).
For performing such radio-frequency oscillation, a circuit comprising a crystal AT resonator and a phase locked loop, and a circuit comprising a SAW resonator are used.
However, the above-described circuits are deficient for performing stable radio-frequency oscillation. For example, a communication-error rate is increased when they are mounted on a communication device.
The details of such problems will now be described.
Circuit Comprising Crystal AT Resonator and PLL
This circuit uses a crystal AT resonator that resonates with stability in the passband from several MHz to a few tens of MHz. FIG. 7(a) illustrates this circuit. As shown in this drawing, a crystal AT resonator 7a, a phase comparator 7b, a low-pass filter 7c, a VCO 7d, and a frequency divider 7e form a PLL circuit. This circuit outputs a signal, obtained by multiplying an oscillated frequency of the crystal AT resonator 7a, from the VCO 7d, thereby achieving the radio-frequency oscillation.
However, in general, Performance of the phase noise characteristics and the jitter characteristics of a ring oscillator that is used for the VCO 7d for performing radio-frequency output are low. Therefore, Performance of the frequency stability of a signal output from this circuit becomes low.
Circuit Comprising SAW Resonator
A SAW resonator uses the property of energy such that energy is concentrated and propagated near the surface of an elastic body. More specifically, the SAW resonator comprises interdigital excitation electrodes and a ladder reflector that are placed on a piezoelectric substrate. A surface wave that is excited by the excitation electrodes is reflected by the reflector, whereby a standing wave is generated. A resonated frequency of the SAW resonator is in the range from a few tens of MHz to several GHz. By using the SAW resonator, a radio-frequency signal can be obtained directly. This circuit is illustrated in FIG. 7(b).
In such a circuit comprising the above-described SAW resonator, the characteristics of the SAW resonator dominate the characteristics of the output signal. However, the SAW resonator has frequency-temperature characteristics such that the resonance frequency varies according to the variation of the ambient temperature. Further, the SAW resonator has a characteristic such that the resonance frequency varies according to fluctuations in power-supply voltage supplied to the circuit and a secular variation. Therefore, the frequency stability of the output signal of the circuit comprising the above-described SAW resonator is decreased by temperature or the like.
To this end, a temperature-compensated circuit comprising a variable capacity diode or the like can be provided separately for providing temperature compensation for the SAW resonator. However, since this method is effective only for the temperature compensation, the frequency stability cannot be raised.
For solving such problems, the object of the present invention is to provide a temperature-compensated radio-frequency oscillator formed as an oscillator using a SAW resonator that achieves high frequency stability and can output a clock signal having a frequency that is constant over a wide temperature range. Another object of the present invention is to provide a communication device using the temperature-compensated radio-frequency oscillator.
The configuration of the temperature-compensated radio-frequency oscillator provided for solving the above-described problems according to the present invention is described below.
The temperature-compensated radio-frequency oscillator is an oscillator for oscillating a SAW resonator, comprising:
a voltage-controlled SAW oscillator that outputs a signal at a frequency that is changed according to a supplied control voltage,
a temperature-compensated oscillator that outputs a signal at a constant frequency independent of the ambient temperature,
a frequency divider that divides the output signal of the voltage-controlled SAW oscillator,
a phase comparator that compares the phase of the output signal of the frequency divider with the phase of the output signal of the temperature-compensated oscillator and outputs a phase difference signal according to the comparison result, and
a low-pass filter for smoothing the phase difference signal,
wherein a phase locked loop is formed by supplying the output signal of the low-pass filter to the voltage-controlled SAW oscillator as the control voltage, and
wherein the oscillation output of the voltage-controlled SAW oscillator is output to the outside.
According to the above-described configuration, the voltage-controlled oscillator outputs a signal at a frequency according to a supplied controlled voltage, and the temperature-compensated oscillator outputs a signal at a constant frequency independent of the ambient temperature. Then, the phase-comparator outputs the phase-difference signal according to the phase difference between the signals output from these circuits. The low-pass filter smoothes the phase-difference signal and outputs the smoothed phase-difference signal to the voltage-controlled SAW oscillator as a control signal. Thus, the temperature-compensated radio-frequency oscillator is formed as a PLL circuit. Therefore, the frequency of the signal output from the voltage-controlled SAW oscillator is kept constant even though the ambient temperature varies.
Preferably, a temperature-compensated crystal oscillator (TCXO) using a crystal AT resonator is used for the temperature-compensated oscillator. By using the TCXO, a high performance, compact temperature-compensated oscillator can be provided at lower cost.
The configuration of the temperature-compensated radio-frequency oscillator according to the present invention is described below.
The temperature-compensated radio-frequency oscillator is an oscillator for oscillating a SAW resonator, comprising:
a voltage-controlled SAW oscillator that outputs a signal at a frequency that is changed according to a supplied control voltage,
a temperature-compensated oscillator that outputs a signal at a constant frequency independent of the ambient temperature,
a frequency divider that divides the output signal of the voltage-controlled SAW oscillator,
a phase comparator that compares the phase of the output signal of the frequency divider with the phase of the output signal of the temperature-compensated oscillator and outputs a phase difference signal according to the comparison result, and
a DSP (Digital Signal Processor) for generating a signal corresponding to the phase difference signal,
wherein a phase locked loop is formed by supplying the signal generated by the DSP to the voltage-controlled SAW oscillator as the control signal, and
wherein the oscillation output of the voltage-controlled SAW oscillator is output to the outside.
According to the configuration, in the phase locked loop, it becomes possible to integrate a low-pass filter (a loop filter) into an IC. Therefore, the size of the circuit is reduced and the resistance to external noise is increased.
The above-described temperature-compensated radio-frequency oscillators that are formed according to the above-described configurations can be provided in a communication device.